106 



GENERAL BOTANY 



vascular bundles, form a complete cylinder separating the pith 

 from the cortex and comparable to the thicker phloem and 



xylem cylinder of 

 Skeletal tissue 

 ^-Epidermis 

 t-Cortex 

 ^Cambium 

 'Xylem 



"-Pith 



woody plants. 



The structure of 

 the young stem 

 of Salvia outlined 

 above is very simi- 

 lar to that observed 



Skeletal 

 tissue 

 Epidermis 

 Cortex 

 Phloem 

 Cambium 



-Pith 



in young, growing 

 stems of herbaceous 

 and woody plants. 

 In some herbaceous 

 plants the vascu- 

 lar bundles remain 

 completely isolated 

 throughout the life 

 of the plant, even 

 in older portions 

 of the stem, and 

 within these bun- 

 dles very little sec- 

 ondary growth of 

 xylem and phloem 

 takes place from 

 the cambium. In 

 older stems of the 

 Salvia (Fig. 56, 5) a 

 solid vascular ring 

 of xylem, phloem, 

 and cambium is to 

 be seen, resembling 

 closely the thicker 

 vascular cylinder of woody plants. This complete cylinder of 

 vascular tissue in the older stems of Salvia is produced by the 

 cambium, which, as the stem matures, forms across the fibrous 

 tissue bands separating the primary bundles, and starts the 



FIG. 56. Structure of the young and mature stem 

 of a common sage (Salma) 



a, a transverse section of a young stem of Salvia. The sec- 

 tion illustrates the structure of herbaceous stems as com- 

 pared with wood stems, like that of the alder and oak ; 

 6, a transverse section of an older stem of Salvia. Secon- 

 dary thickening by means of the cambium has taken 

 place, forming a wider zone of phloem and xylem 



